1. Field of the Invention
The invention relates generally to an optical communication system using a high altitude tethered balloon for high data rate communication between ground-based and space-based or high altitude-based instrumentation.
2. Discussion of the Related Art
Optical communication systems operate at very high frequencies and are ideally suited for high bandwidth applications. Compared to radio frequency (RF) communication systems, optical communication systems also generally permit smaller and lighter system components, require less power, and are more secure. However there are significant problems associated with the operation of optical communication systems in the atmosphere. Clouds, rain and fog can scatter optical beam energy and disrupt communications. Furthermore, turbulence in the atmosphere can distort the optical beam and also cause lost communications.
Many different techniques have been proposed to mitigate problems associated with atmospheric optical communications. One approach, when communicating from a ground station to an overhead satellite, is to have several ground stations at different locations so that a transmission can be sent from the ground station that is least obstructed by clouds. However such an approach likely requires more than three stations, separated by more than 200 km, to provide a reliable transmission capability. The costs associated with this approach are prohibitive.
Another approach is to send a repeated packet of data toward a receiver. If a packet is received during a brief “scintillation window” that results in a clear line-of-sight between the transmitter and the receiver, an acknowledgement is returned to the transmitter. A subsequent packet of data is then repeatedly broadcast until it is received. However this approach is only able to establish communications through the atmosphere for very brief periods, thus high data rate communications are not optimized.
Tethered balloons have been used for generations for long distance communications. Historically the balloons relayed information, often simply visual observations, from one point on the ground to another. Ground-to-aircraft and ground-to-spacecraft RF communications are also relayed using balloons. However viable ground-to-spacecraft optical communications relayed via tethered balloons have not been suggested. Such balloon-relayed optical communications require overcoming both a) the above mentioned problem of atmosphere-induced optical scatter and distortion; and b) the problem of pointing and stabilizing transmitting and receiving optics in turbulent air to achieve the stable narrow beam alignment necessary for modern optical communications.
To overcome atmosphere induced scatter and distortion of optical signals, another solution is to effectively move above the atmosphere. Free flying high altitude scientific balloons have been used successfully for many years; however, very high altitude tethered balloons operating for extended periods of time in the stratosphere above much of the earth's weather have been generally unsuccessful. Only recently have practical techniques been suggested enabling tethered balloon flights at altitudes near 20 km. FIG. 1 illustrates a wind profile and ascent trajectory involved in modulating a tethered balloon through peak jet stream winds to high altitude. During such an ascent maximum tether tension occurs at the balloon-tether interface while the balloon is ascending through the range of peak winds between 8–12 km. A thorough understanding of such balloon ascent conditions is necessary to optimize the numerous parameters such as downwind displacement, tether length, free lift, balloon size, and tether and balloon materials.
Pointing and stabilizing optic communications equipment attached to a balloon is also a difficult technical challenge. At lower altitudes, the wind forces can be so severe that stable alignment of an optical transceiver is not practical.
In summary, known means of transmitting and receiving high data rate optical communications between ground and space have numerous disadvantages. No solutions have been proposed to efficiently overcome the transmission problems associated with cloud cover and atmospheric disturbances. The advantages however of optical communications are significant compared with other long distance communication methods. The high bandwidth facilitated by optical communications is becoming increasingly important in global communications. Therefore there is a need for an optical communication system that overcomes the above difficulties of optical transmission through the atmosphere.